Experimental and modeling study of H2S formation and evolution in air staged combustion of pulverized coal

High-concentration H₂S formed in the reduction zone of pulverized coal air-staged combustion can result into the high temperature corrosion of water wall tube of boiler, so it is of great importance to accurately predict H₂S concentration for the safe operation of boilers and burners. H₂S formation and evolution depends on two steps: the sulfur release from coal conversion and gas-phase reactions of sulfur species. In this study, the sulfur release characteristics from the pyrolysis of 17 coals, including 5 lignite, 9 bituminous coals and 3 anthracites, are investigated in a drop tube furnace (DTF). Sulfur release model is developed to describe the relationship between sulfur release and coal types. A global gas-phase reaction mechanism of sulfur species composed of ten reactions is used to calculate and predict the formation and evolution of H₂S, COS and SO₂ in the reduction zone of pulverized coal air-staged combustion. A wide range of air-staged combustion experiments of 17 coals are conducted in the DTF at different temperatures and stoichiometric ratios to validate the developed model. The results show that the prediction errors of sulfur species, including SO₂, H₂S and COS, are within ± 30%, which indicates that the developed prediction model of sulfur species is of great assistance for CFD modeling of actual engineering application.     

Measurements and modelling of oxy-fuel coal combustion

Oxy-fuel combustion is one of the most promising technologies to isolate efficiently and economically CO₂ emissions in coal combustion for the ready carbon sequestration. The high proportions of both H₂O and CO₂ in the furnace have complex impacts on flame characteristics (ignition, burnout, and heat transfer), pollutant emissions (NO_x, SO_x, and particulate matter), and operational concerns (ash deposition, fouling/slagging). In contrast to the existing literature, this review focuses on fundamental studies on both diagnostics and modelling aspects of bench- or lab-scale oxy-fuel combustion and, particularly, gives attention to the correlations among combustion characteristics, pollutant formation, and operational ash concerns. First, the influences of temperature and species concentrations (e.g., O₂, H₂O) on coal ignition, volatile combustion and char burning processes, for air- and oxy-firing, are comparatively evaluated and modelled, on the basis of data from optically-accessible set-ups including flat-flame burner, drop-tube furnace, and down-fired furnace. Then, the correlations of combustion-generated particulate/NO_x emissions with changes of combustion characteristics in both air and oxy-fuel firing modes are summarized. Additionally, ash deposition propensity, as well as its relation to the formation of fine particulates (i.e. PM_0.2, PM₁ and PM₁₀), for both modes are overviewed. Finally, future research topics are discussed. Fundamental oxy-fuel combustion research may provide an ideal alternative for validating CFD simulations toward industrial applications.     

Predictive one step kinetic model of coal pyrolysis for CFD applications

This work’s aim is the development of a simplified kinetic model for coal devolatilization, suitable for CFD applications. The detailed model of coal devolatilization, already developed and tested against a very large set of experiments and conditions, is too large to be implemented and used inside a CFD code.An automatic procedure is developed to estimate the kinetic parameters and stoichiometric coefficients of a one step model. An optimization technique manages to minimize an objective function which measures the distance between the simple one step kinetic mechanism and the results of the detailed model of coal volatilization. The results show that rate parameters can be expressed as function of the coal rank, or better of its carbon content. Despite its simplicity, the one step model is able to characterize not only the weight loss, but also the relative yields of gaseous species (CO, H₂O, CO₂, H₂, C₂H₄, HCN, H₂S), tar species and char. The coal database discussed in this work includes 13 coals of different elemental composition, from lignite to anthracite. The agreement of the one step model with the detailed model of coal volatilization is satisfactory for both evolution rates and cumulative values. Finally, it is important to newly underline that the proposed model, based on the detailed model, is predictive and only needs the coal elemental composition (coal rank) as an input. Two sets of correlation for the distribution of the nitrogen and sulfur compounds into solid, tar and gas phases are also proposed.     

Catalytic effect of water,water dimer,water trimer,HCOOH, H2SO4, CH3CH2COOH and HN(NO2)2 on the isomerisation of HN(NO2)2 to O2NNN(O)OH: a mechanism study

In this article, the isomerisation mechanisms of HN(NO₂)₂ to O₂NNN(O)OH without and with catalyst X (X = H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄, CH₃CH₂COOH and HN(NO₂)₂) have been investigated theoretically at the CBS-QB3 level of theory. Our results show that the catalyst X (X = H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄ and CH₃CH₂COOH) shows different positive catalytic effects on reducing the apparent activation energy of the isomerisation reaction processes. Such different catalytic effects are mainly related to the number of hydrogen bonds and the size of the ring structure in X (X = H₂O, (H₂O)₂ and (H₂O)₃)-assisted transition states, as well as different values of pK_a for H₂SO₄, HCOOH and CH₃CH₂COOH. Very interesting is also the fact that H₂SO₄-assisted reaction is the most favourable for the hydrogen transfer from HN(NO₂)₂ to O₂NNN(O)OH, due to the smallest pK_a (?3.0) value of H₂SO₄ than H₂O, HCOOH, H₂SO₄ and CH₃CH₂COOH, and also because of the largest ∠X???H???Y (the angle between the hydrogen bond donor and acceptor) involved in H₂SO₄-assisted transition state. Compared to the self-catalysis of the isomerisation mechanisms of HN(NO₂)₂ to O₂NNN(O)OH, the apparent activation energy of H₂SO₄-assisted channel also reduces by 9.6 kcal?mol^?1, indicating that H₂SO₄ can affect the isomerisation of HN(NO₂)₂ to O₂NNN(O)OH, most obvious among all the catalysts H₂O, (H₂O)₂, (H₂O)₃, HCOOH, H₂SO₄, CH₃CH₂COOH and HN(NO₂)₂.     

Optical diagnostics on coal ignition and gas-phase combustion in co-firing ammonia with pulverized coal on a two-stage flat flame burner

Gradual substitution of coal with green ammonia is a practical approach for the coal power phasedown at a minimal cost of modification, but the ignition and gas-phase reaction during co-firing NH₃ with coal remain largely unclear. In this work, we investigate the co-combustion behaviors of NH₃ and a high-volatile coal on a two-stage flat flame burner. Remarkably, the post-flame oxygen mole fraction X_i,O2 of the inner stage can be manipulated to reproduce a proper reducing-to-oxidizing environment that coal particles experience in the practical combustor. We first reveal that, under certain values of X_i,O2 and NH₃ co-firing energy ratios E_NH3, the reaction intensity (manifested by OH-PLIF signals) in the NH₃-coal flame is stronger than burning either pure coal or NH₃. This synergetic effect originates from an NH₃-combustion-induced enhancement of volatile release. We then propose a characteristic time scale τ_OH from the OH signals for the initiation of overall reactions in the system. In the case of X_i,O2=0, τ_OH monotonically increases with E_NH3, while for X_i,O2=0.2, the trend transitions to a decreasing one. It can be interpreted by comparing τ_OH with the characteristic O₂ diffusion time, coal particle heating time, and the coal pyrolysis time under different X_i,O2. Furthermore, the coal particle ignition in coal-NH₃ flames can no longer be determined by visual images. Instead, we apply CH* chemiluminescence to identify the stages of coal particle ignition and volatile combustion in the NH₃-coal flame. While NH₃ addition has both positive (elevating temperatures & diluting coal particles) and negative (consuming O₂) effects on coal ignition, the combined influence of E_NH3 is marginal on coal ignition delay time. On the other hand, the volatile combustion time decreases linearly with E_NH3, suggesting a pure effect of reduced coal feed rate.     

Effect of alkali metals on nitrogen oxide emission: Role of Na and its occurrence in coal

High-alkali coal contains relatively high contents of alkali metals, which can be usually released in the form of gaseous chlorides and hydroxides during combustion. The effect of alkali metals on NO formation is analyzed in an electrical heated drop-tube furnace at 800–1200 °C during coal combustion. Based on experiments and simulations, the mechanisms underlying the effects of Na salts on NO emission are clarified in CO/NH₃/O₂/H₂O/Na additive (NaCl, Na₂SO₄, and NaAc) systems. The results indicate that the yield of NO initially increases and then decreases as the furnace temperature increased. As the temperature increased from 800 to 1000 °C, NO precursors (HCN and NH₃) undergo accelerated oxidation to form NO. When the furnace temperature is greater than 1000 °C, due to the rapid precipitation of volatiles, a local reducing atmosphere is present around the pulverized coal particles, which inhibits NO formation. NaCl and NaAc addition significantly inhibit NO formation. However, the inhibitory effect is weakened at higher temperatures (>1000 °C). The Na₂SO₄ additive exerts little effect on NO generation during combustion because of its stable chemical properties. The same conclusion is also obtained from gaseous experiments showing that NaCl and NaAc significantly inhibit NH₃ oxidation to form NO. Based on the results of calculations, NaCl and NaAc addition inhibits NO formation by promoting the recombination of H, O and OH and reducing the concentrations of radicals. According to the analysis of chemical reactions, the effect of NaCl and NaAc on NO formation is mainly determined by the competitive relationships among multiple reactions.     

Realization of oxyfuel combustion for near zero emission power generation

Oxyfuel combustion is one of the promising carbon capture and storage (CCS) technologies for coal-fired boilers. In oxyfuel combustion, combustion gas is oxygen and recirculating flue gas (FGR) and main component of combustion gas is O₂, CO₂ and H₂O rather than O₂, N₂ in air combustion. Fundamental researches showed that flame temperature and flame propagation velocity of pulverized cloud in oxyfuel combustion are lower than that in air with the same O₂ concentration due to higher heat capacity of CO₂. IHI pilot combustion test showed that stable burner combustion was obtained over 30% O₂ in secondary combustion gas and the same furnace heat transfer as that of air firing at 27% O₂ in overall combustion gas. Compared to emissions in air combustion, NOx emission per unit combustion energy decreased to 1/3 due to reducing NOx in the FGR, and SO_x emission was 30% lower. However SO_x concentration in the furnace for the oxyfuel mode was three to four times greater than for the air mode due to lower flow rate of exhaust gas. The higher SO₃ concentration results that the sulphuric acid dew point increases 15–20 °C compared to the air combustion. These results confirmed the oxyfuel pulverized coal combustion is reliable and promising technology for coal firing power plant for CCS.In 2008, based on R&D and a feasibility study of commercial plants, the Callide Oxyfuel Project was started in order to demonstrate entire oxyfuel CCS power plant system for the first time in the world. The general scope and progress of the project are introduced here. Finally, challenges for present and next generation oxyfuel combustion power plant technologies are addressed.     

Investigation of sulfur speciation in particles from small coal-burning boiler by XANES spectroscopy

Sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy was employed to study the speciation of sulfur in raw coal, ash by-product and fine particulate matter from a small coal-burning boiler. By means of least square analysis of the XANES spectra, the major organic and inorganic sulfur forms were quantitatively determined. The results show that about 70% of the sulfur in raw coal is present as organic and a minor fraction of the sulfur occurs as other forms: 17% of pyrite and 13% of sulfate. While in bottom ash, fly ash, and PM_2.5, the dominant form of sulfur is sulfate, with the percentage of 80,79 and 94, respectively. Moreover, a number of other reduced sulfur including thiophenic sulfur, element sulfur and pyrrhotite are also present. During coal combustion, most of organic sulfur and pyrite were oxidized and released into the atmosphere as SO₂ gas, part of them was converted to sulfate existing in coal combustion by-products, and a small part of pyrite was probably reduced to elemental sulfur and pyrrhotite. The results may provide information for assessing the pollution caused by small boiler and developing new methods for the control of SO₂ pollution.     

The transformation and fate of sulphur during CO2 gasification of a spent tyre pyrolysis char

The transformation and fate of sulphur (S) in a spent tyre pyrolysis char during CO₂ gasification were studied by following the S species and contents using X-ray photoelectron spectroscopy (XPS). The spent tyre pyrolysis char (particle size fraction ≤150 µm), without and with 1 M HCl acid washing to remove inorganic S, were gasified in a fixed bed reactor. The effect of temperature (850, 950, 1050 °C), reaction time (1, 2, 3, 6 h) and CO₂ concentration (33.3, 50.0, 66.7 vol% in N₂) on the S species in the char samples were investigated. The main S species in the spent tyre pyrolysis char were ZnS and aliphatic sulphide. After CO₂ gasification, aliphatic sulphide, thiophene, sulphoxide and sulphone became the dominant organic S while ZnS and CaSO₄ were the main inorganic S. The percentage of total S increased with increasing gasification temperature, time and CO₂ concentration. The content of organic S increased with increasing gasification temperature and time, while, the content of inorganic S decreased. Increasing CO₂ concentration had negligible effect on the content of organic S but led to significant reduction in the content of inorganic S since ZnS reacted with CO₂ to produce ZnO and SO₂. Aliphatic sulphide, sulphoxide and sulphone were shown to have transformed to more stable thiophene. ZnS decomposed to release S_X at > 900 °C while CaSO₄ reacted with CO and carbon to produce COS. Both S_X and COS reacted with the organic matrix in the char to form sulphoxide and sulphone.     

A reactive molecular dynamics study of NO removal by nitrogen-containing species in coal pyrolysis gas

Coal splitting and staging is a promising technology to reduce nitrogen oxides (NOx) emissions from coal combustion through transforming nitrogenous pollutants into environmentally friendly gasses such as nitrogen (N₂). During this process, the nitrogenous species in pyrolysis gas play a dominant role in NOx reduction. In this research, a series of reactive force field (ReaxFF) molecular dynamics (MD) simulations are conducted to investigate the fundamental reaction mechanisms of NO removal by nitrogen-containing species (HCN and NH₃) in coal pyrolysis gas under various temperatures. The effects of temperature on the process and mechanisms of NO consumption and N₂ formation are illustrated during NO reduction with HCN and NH₃, respectively. Additionally, we compare the performance of NO reduction by HCN and NH₃ and propose control strategies for the pyrolysis and reburn processes. The study provides new insights into the mechanisms of the NO reduction with nitrogen-containing species in coal pyrolysis gas, which may help optimize the operating parameters of the splitting and staging processes to decrease NOx emissions during coal combustion.     

Oxidation of hydrogen sulfide and corrosion of stainless steel in gas mixtures containing H<Subscript>2</Subscript>S,O<Subscript>2</Subscript>, H<Subscript>2</Subscript>O,and CO<Subscript>2</Subscript>

The composition of volatile and solid products of oxidation of hydrogen sulfide and stainless steel in gas mixtures containing H₂S, O₂, H₂O, and CO₂ has been determined using mass spectrometry, x-ray diffraction analysis, and scanning electron microscopy. It has been shown that holding an H₂S–O₂ mixture at 301 K results in prevailing formation of elemental sulfur and iron sulfides in the form of porous hygroscopic crust on the reactor wall surface. Formation of gas-phase sulfur causes self-acceleration of the oxidation of hydrogen sulfide; the resulting water triggers corrosion of the reactor wall. Heating of the resulting sulfur-sulfide crust in O₂ medium is accompanied by formation of SO₂ and heat release at T > 508 K. After heating of the H₂S–CO₂ mixture to 615 K, H₂ and COS were found in the volatile reactants; no noticeable corrosion of the reactor wall has been detected. It has been established that addition of O₂ to the H₂S–CO₂ mixture and its heating to 673 K leads to formation of ferrous sulfates. The mechanisms of the observed processes are discussed.     

Interaction mechanism among CO,H2S and CuO oxygen carrier in chemical looping combustion: A density functional theory calculation study

When using coal-derived syngas or coal as fuel in chemical looping combustion (CLC), CO as a representative pyrolysis/gasification product and H₂S as the main sulfurous gas coexist in fuel reactor. Either CO or H₂S can absorb on the surface of CuO (the active component of Cu-based oxygen carriers), and reactions will occur among them. In this study, density functional theory (DFT) calculations are conducted to investigate the interaction among H₂S, CO, and CuO, including: the reaction between CO and H₂S over CuO particle, the influence of CO on the H₂S dissociation and further reaction process, and the impact of H₂S dissociation products on CO oxidation. Firstly, the co-adsorption results suggest that H₂S might directly react with CO to produce COS via the Eley–Rideal mechanism, while CO prefers to react with HS* or S* via the Langmuir–Hinshelwood mechanism. This means that the reaction mechanisms between CO and H₂S will change as the H₂S dissociation proceeds, which has already been forecasted by the co-adsorption energies and verified by all of potential Eley–Rideal and Langmuir–Hinshelwood reaction pathways. Then, the influence of CO on the H₂S dissociation process is examined, and it is noted that the presence of CO greatly limits the dissociation of H₂S due to the increased energy barrier of the rate-determining dehydrogenation step. Furthermore, the impact of H₂S dissociation products on CO oxidation by CuO is also investigated. The presence of H₂S and S* significantly supresses the CO oxidation activity, while the presence of HS* slightly promotes the CO oxidation activity. Finally, the complete interaction mechanisms among H₂S, CO, and CuO are concluded. It should be noted that COS will be inevitably produced via the Langmuir–Hinshelwood reaction between surface S* and CO*, which is prior to H₂O generation and subsequent sulfidation reaction.     

Total cross sections for electrons scattering from simple molecules containing the larger atom sulfur at 30-5000eV

A complex optical model potential modified by the concept of bonded atom, which takes into consideration the overlapping effect of electron clouds, is employed to calculate the total cross sections for electrons scattering from simple molecules （SO2, H2S, OCS, CS2 and SO3） containing the larger atom, sulfur, at 30-5000eV by using the additivity rule model at Hartree-Fock level. The quantitative molecular total cross section results are compared with those obtained in experiments and other calculations wherever available, and good agreement is obtained. It is shown that the additivity rule model together with the complex optical model potential modified by the concept of bonded atom can give the results closer to the experiments than the one unmodified by it. So, the introduction of bonded-atom concept in complex optical model potential betters the accuracy of the total cross section calculations of electrons from the molecules containing the larger atom, sulfur.     

In situ parametric study of alkali release in pulverized coal combustion: Effects of operating conditions and gas composition

This work concerns a parametric study of alkali release in a lab-scale, pulverized coal combustor (drop tube reactor) at atmospheric pressure. Measurements were made at steady reactor conditions using excimer laser fragmentation fluorescence (ELIF) and with direct optical access to the flue gas pipe. In this way, absolute gas-phase alkali species could be determined in situ, continuously, with sub-ppb sensitivity, directly in the flue gas. A hard coal was fired in the range 1000–1300 °C, for residence times in the range 3–5 s and for air numbers λ (air/fuel ratios) from 1.15 to 1.50. In addition, the amount of chlorine, water vapor and sulfur, respectively, was increased in known amounts by controlled dosing of HCl, H₂O and SO₂ into the combustion gas to determine effects of these components on release or capture of the alkali species. The experimental results are also compared with values calculated using ash/fuel analyses and sequential extraction to obtain a fuller picture of alkali release in pulverized fuel combustion.     

A reduced mechanism for primary reactions of coal volatiles in a plug flow reactor

In the present paper, the authors study the primary reactions of coal volatiles and a detailed mechanism has been made for three different environments: thermal decomposition (pyrolysis), partial oxidation (O₂) and O₂/CO₂ gasification in a plug flow reactor to analyze the combustion component. The computed results have similar trend for three different environments with the experimental data. A systematically reduced mechanism for O₂/CO₂ gasification has also been derived by examination of Rate of Production (ROP) analysis from the detailed mechanism (255 species and 1095 reactions). The reduced mechanism shows similar result and has been validated by comparing the calculated concentrations of H₂, CH₄, H₂O, CO, CO₂ and polycyclic aromatic hydrocarbon (PAH) with those of the detailed mechanism in a wide range of operating conditions. The authors also predicted the concentration profiles of H₂, CO, CO₂ and PAH at high temperature and high pressure.     

Kinetics of C<Subscript>10</Subscript>H<Subscript>7</Subscript>Br Pyrolysis

The temperature and pressure-dependent rate constants for the process C₁₀H₇Br ? C₁₀H₇+Br were evaluated using the variable reaction coordinate transition state theory VRC-TST. The calculated rate constants and computational fluid dynamics (CFD) calculations were employed to estimate the pyrolysis efficiency of 2-bromonaphthalene in the resistively-heated SiC high-temperature “chemical reactor” at the temperature of about 1500 K. The observed 40% pyrolysis efficiency is reproduced by CFD calculations if the value of the calculated rate constant for the C₁₀H₇Br pyrolysis is increased by a factor of 2.     

Numerical study on K/S/Cl release during devolatilization of pulverized biomass at high temperature

In this paper, the interaction between different organic and inorganic K/S/Cl compounds in the solid structure of biomass is studied and a model is presented to predict the temporal release of K_g, HCl, CH₃Cl, KCl, KOH, K₂SO₄ and SO₂ from biomass devolatilization. Four types of pulverized biomass are chosen from literature, two of which have no chlorine content and two with chlorine content in lower stoichiometry to potassium. The results of the model are compared with the experimental measurements. In the presence of chlorine, KCl, HCl and K_g were found to be the dominant chlorine and potassium species. In the absence of chlorine, K_g dominates the release of potassium. KOH and K₂SO₄ release into the gas phase towards the end of devolatilization due to the overlapping with char combustion. SO₂ is the main sulfur species released into the gas phase. The model is coupled with a CFD solver where the gas phase chemistry of the K/S/Cl system can be studied using available chemical mechanisms for these species.     

The combined effect of H2O and SO2 on CO2 uptake and sorbent attrition during fluidised bed calcium looping

The effect of steam and sulphur dioxide on CO₂ capture by limestone during calcium looping was studied in a novel lab-scale twin fluidised bed device (Twin Beds – TB). The apparatus consists of two interconnected batch fluidised bed reactors which are connected to each other by a duct permitting a rapid and complete pneumatic transport of the sorbent (limestone) between the reactors. Tests were carried out under typical calcium looping operating conditions with or without the presence of H₂O and/or SO₂ during the carbonation stage. Carbonation was carried out at 650°C in presence of 15% CO₂, 10% steam (when present) and by investigating two SO₂ levels, representative of either raw (1500?ppm) or pre-desulphurised (75?ppm) typical flue gas derived from coal combustion. The sorbent used was a reactive German limestone. Its performance was evaluated in terms of CO₂ capture capacity, sulphur uptake, attrition and fragmentation. Results demonstrated the beneficial effect of H₂O and the detrimental effect of SO₂ on the CO₂ capture capacity. When both species were simultaneously present in the gas, steam was still able to enhance the CO₂ capture capacity even outweighing the negative effect of SO₂ at low SO₂ concentrations. A clear relationship between degrees of Ca carbonation and sulphation was observed. As regards the mechanical properties of the sorbent, both H₂O and SO₂ hardened the particle surface inducing a decrease of the measured attrition rate, that was indeed always very low. Conversely, the fragmentation tendency increased in presence of H₂O and SO₂ most likely due to the augmented internal stresses within the particles. Clear bimodal particle size distributions for in-bed sorbent fragments were observed. Microstructural scanning electron microscope and porosimetric characterisations aided in explaining the observed trends.     

A deep insight into arsenic adsorption over γ-Al2O3 in the presence of SO2/NO

Arsenic is easily evaporated during coal combustion, which not only raises serious environmental concerns but also results in the deactivation of catalyst in selective catalytic reduction (SCR) systems. It is a promising method to use sorbents for the capture of arsenic vapors (As₂O₃(g)) before As-containing flue gas entering SCR catalyst. However, arsenic has a strong affinity with sulfur in coal and SO₂ in the coal combustion flue gas strongly suppresses As₂O₃(g) capture by typical Ca/Fe-based sorbents. This study estimated the selective capture of As₂O₃(g) by γ-Al₂O₃ and the effects of SO₂ and NO on the arsenic adsorption were investigated. The results showed that As₂O₃(g) adsorption over γ-Al₂O₃ was effectively conducted at temperatures ranging from 300 to 400 °C. In the reacted γ-Al₂O₃, arsenic was predominantly in the form of As³⁺ through reactions with Al-O bonds and positive charged alumina ions. SO₂ was slightly adsorbed on γ-Al₂O₃, which had a limited effect on arsenic adsorption. The adsorption of SO₂ on γ-Al₂O₃ mainly occurred on the sites of hydroxyl groups (Al-OH) and few adsorbed SO₂ was bound with positive charged alumina ions. NO was catalytically oxidized by γ-Al₂O₃ and released as NO₂. Nevertheless, NO competed with As₂O₃(g) to adhere to positive charged alumina ions and strongly suppressed arsenic adsorption over γ-Al₂O₃. Fortunately, in the presence of SO₂, NO was mostly transformed into intermediate (-SO₃NO) at the sites of Al-OH on γ-Al₂O₃. As a result, the adverse effect of NO on the adsorption of As₂O₃(g) was weakened.     

Modelling alkali metal emissions in large-eddy simulation of a preheated pulverised-coal turbulent jet flame using tabulated chemistry

The numerical modelling of alkali metal reacting dynamics in turbulent pulverised-coal combustion is discussed using tabulated sodium chemistry in large eddy simulation (LES). A lookup table is constructed from a detailed sodium chemistry mechanism including five sodium species, i.e. Na, NaO, NaO₂, NaOH and Na₂O₂H₂, and 24 elementary reactions. This sodium chemistry table contains four coordinates, i.e. the equivalence ratio, the mass fraction of the sodium element, the gas-phase temperature, and a progress variable. The table is first validated against the detailed sodium chemistry mechanism by zero-dimensional simulations. Then, LES of a turbulent pulverised-coal jet flame is performed and major coal-flame parameters compared against experiments. The chemical percolation devolatilisation (CPD) model and the partially stirred reactor (PaSR) model are employed to predict coal pyrolysis and gas-phase combustion, respectively. The response of the five sodium species in the pulverised-coal jet flame is subsequently examined. Finally, a systematic global sensitivity analysis of the sodium lookup table is performed and the accuracy of the proposed tabulated sodium chemistry approach has been calibrated.